1. Field of the Invention
The present invention relates to an etching apparatus, in particular an etching apparatus for control of the temperature depending on the operational status.
2. Description of the Prior Art
Reactive ion beam etching (RIE) apparatuses have been used for the etching pattern formation on substrates such as glass substrates and wafers (i.e., processing objects) in vapor phase. Among them, a plasma etching apparatus performs the etching pattern formation on a substrate placed on an electrode in a chamber using a plasma-generating reactive gas. In the process of etching pattern formation, the temperature of the substrate increases as radicals in the reactive gas come into collision with the surface of the substrate to proceed the chemical reaction thereon. The increase in the temperature of the substrate results in the deformation thereof, so that a trouble will be occurred in the process of micro-fabrication. Conventionally, for avoiding such a deformation, the temperature of the substrate is controlled using a cooling system in which a cooling mechanism is provided on the electrode to be used as a substrate mounting.
Referring now to FIG. 2, the configuration of the conventional cooling system is schematically illustrated. The conventional cooling system 120 is connected to a processing device 110 and is configured to take the heat from a substrate 160 in the device 110. In other words, a refrigerant stored in a refrigerant tank 122 is fed into the inside of an electrode 112 at a predetermined flow rate through a refrigerant-circulating path 121 by means of a pump 124. Then, the refrigerant in the electrode 112 absorbs heat to cool down the substrate 160 on the electrode 112. Subsequently, the heat-absorbing refrigerant is returned into the refrigerant tank 122 through the refrigerant-circulating path 121 and the refrigerant is then cooled down by a first heat exchanger 125 in the refrigerant tank 122, followed by using the refrigerant for cooling the substrate 160 again.
Here, in the refrigerating system 120, a temperature sensor 123 continuously monitors the temperature of the refrigerant in the refrigerant tank 122. A temperature controlling device 130 controls the flow rate of a gaseous refrigerant by a compressor 126 such that the temperature detected by the temperature sensor 123 can be coincident with a predetermined temperature to control the temperature or flow rate of the refrigerant in the refrigerant tank 122 through the first heat exchanger 125.
Regarding the process of etching pattern formation on the substrate 160 by the processing device 110, there is no need to extensively cool the substrate 160 because of no heat accumulation occurred therein before the initiation of such a process. In this case, that is, there is no direct heat load from the plasma to the electrode 112 before the initiation of the process. On the other hand, heat is gradually accumulated in the substrate when the substrate is continuously subjected to the etching process, so that the substrate should be cooled more extensively.
In the conventional cooling system, however, the flow rate of the refrigerant is maintained constant irrespective of whether the process is in an early state or in a final state. Therefore, there is a problem in which the temperature of the electrode is comparatively low when the number of processed substrates is small but it increases as the number of the processed substrates increases.
Regarding such a problem, for example, we measured the difference between the temperature of the substrate at the time of initiating the process of etching pattern formation on the first substrate and the temperature of the 22nd substrate at the time of initiating the process thereon, resulting in a temperature of 5.2° C. (i.e. 30° C. at the time of initiating the first etching process and 35.2° C. at the time of initiating the 22nd etching process). In addition, the difference between the temperature of the substrate at the time of completing the etching process on the first substrate and the temperature of the substrate at the time of completing the etching process on the 22nd substrate was 3.0° C. (i.e., 34.8° C. at the time of completing the first etching process and 37.8° C. at the time of completing the 22nd etching process).
Referring now to FIG. 3, furthermore, there is shown a graph that represents the relationship between the number of processed substrates and the shape difference (i.e., the amount of CD-shift [μm]) between the first substrate and the third or later substrate when the substrates were successively processed. Here, the term “CD-shift” means a value obtained by subtracting the line width of an etching mask before the process of etching pattern formation from the line width of an etching target material (i.e., each of the substrates) after the process. As shown in the figure, when the etching target material was a silicon oxide film (3.8 nm in film thickness), the CD shift was approximately 4 nm (the CD shift of the first one was −36 nm, while the CD shift of the third or later one was −40 nm). Such thermal variations of CD shift may affect on the micro-fabrication of a desired device.
Furthermore, when the processing device is out of action or is in an early state of the process, the cooling is excessively performed because the flow rate of the refrigerant for controlling the temperature of the electrode is substantially over the desired amount thereof. Therefore, there is another problem of the increase in electric power consumption because the flow rate of the refrigerant is maintained constant regardless of the usage pattern of the refrigerant when the device is in action or not.